This is a rather unhelpful message, as it does not specify what requirements it is missing. This is specially considering that their Installation instructions states (https://support.google.com/adsense/answer/3380627?hl=en&ref_topic=3380274):

System requirements

Before you start using the Google Publisher Plugin, make sure you meet the following minimum requirements:

PHP version: minimum 5.2.0

WordPress: minimum 3.0

Ensure that you can install third-party WordPress plugins.

So… what am I missing? Pretty sure I am running version 5.5.3 and WordPress is version 3.8.1

I decided I needed to dive into the code to figure out what it was doing and there it was in the Utils.php file:

If you look at the code you will see all the meetsMinimumRequirementsOrDie function will show the error message if any of the two conditions are met:

The filter, json, pcre and SPL extensions are loaded

The PHP version is 5.2 or above

So now that we know what we need how do we get it?

Well, Filter, PCRE and SPL have been added by default over time to PHP and by version 5.5.3 you can count on them. At the time of writting 5.5.3 is the version that comes when you install php using apt-get install php5 on Ubuntu.

So that leaves us with json. To get it installed and loaded simply do the following:

Install the extension via repository:apt-get install php5-json

Restart your php service:

service php5-fpm restart

And you’re done! If you are using a different version that doesn’t come with the other 3 extensions you could edit the utils.php file to show the $extension that is throwing it off like this: